Screw extruders employing kneading discs of the type to which this invention relates are known. Such kneading discs may be employed in single-shaft or multi-shaft screw extruders wherever a particularly thorough kneading action and shearing of the material to be transported is required. The dimensions of the kneading discs generally conform to similar dimensions in the associated helical screw. For example, in a twin screw extruder, the kneading disc may be of lenticular cross section while it might have a substantially triangular cross section in a triple screw extruder. The peripheral contours of the kneading disc are generally circular sections.
Kneading discs are employed in extruders in locations in which the material to be transported is exposed to the highest shear forces which reach their peak value at the points of the kneading discs which lie at the greatest radial distance from its central axis. These regions of the discs are thus subject to a particularly high degree of wear and tear. The extreme regions may have various shapes and generally have some finite area but are often referred to as kneading edges. The overall resistance of the kneading disc to wear and tear may normally be increased by hardening, either through a nitriding process or by case hardening or by a hardening throughout of the entire component. It is also known in the art to apply to the kneading edges a wear resistant material, for example by welding. These known procedures have a number of disadvantages. When hardenable steels are used as the basic material for the kneading discs, for example steel such as tool steels or high speed steels, welded joints are virtually impossible to produce. Furthermore, any welding process involves a heating of adjacent areas to an extent that they lose the hardening acquired and thus lose their resistance to wear and tear. Furthermore, the fabrication and machining of hardened workpieces is extremely difficult and expensive. In nitrided or case-hardened, i.e., surface hardened kneading discs, any subsequent welding process also diminishes the degree of hardening in the region of heating and thus tends to destroy the resistance to wear and tear in that region. If, on the other hand, the wear-resistant material is welded onto the kneading body prior to hardening, the diffusion processes taking place during the surface hardening are very uneven and non-uniform. Furthermore, when the kneading disc is hardened by quenching for example, the welded-on wear resistant materials may pop off due to the bursting of the welded seams.
The aforementioned disadvantages are inherent, for example, in a known extruder machine such as described by U.S. Pat. No. 3,375,549 in which a wear-resistant material is welded to the flights of the helix in an extruder machine.
From the German Auslegeschrift, No. 23 35 588, it is known to produce compound castings with an armor plating of particles of a very hard material in the zones which are to made wear-resistant. The manner of production in that method is to embed the particles of the hard and wear-resistant material in the required areas prior to casting. Accordingly, the individual particles of hard material must be located and held in the mold in a proper orderly distribution which is quite difficult to achieve.
The German Auslegeschrift No. 17 59 599 proposes to protect the tips of mixing blades in, for example, rotary mixing troughs with a plate of a hard metal, somewhat in the manner as is customary for metal cutting machinery, for example lathes, milling machines and the like.
A proposal to protect the interior surfaces of the housing containing the helical screws of screw extruders by means of wear-resistant inserts is described in U.S. Pat. No. 4,036,540.